During the recent total solar eclipse in the US, I overlapped my hands, using the gaps between my fingers to form 鈥減inholes鈥�. Just before the start of totality, someone noticed that the shadows formed through this 鈥減inhole camera鈥� onto a whiteboard were so sharp that the individual hairs on my arms were visible. How thin must the solar crescent be for the shadows to be this sharp, and how soon before totality would this occur? (Continued)
鈥� Your previous correspondent is correct in stating that a sharp image from a pinhole requires minimal diffraction. But unfortunately diffraction (which occurs at the edges of the aperture of an optical system) becomes greater when the pinhole is smaller. The 鈥渋deal pinhole鈥� he describes would not resolve an image at all because it would function as a wave source.
Indeed, diffraction is a real problem in photography, particularly with digital cameras, in which the blurring effect is magnified by the small pixel size. This is one of the reasons why having more pixels does not necessarily make better cameras.
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鈥淭he sharpness of any shadow depends on the size of the light source鈥�
The previous answer was correct inasmuch as the resolution of the image is not affected, but the size of the light source does affect contrast, and therefore the perceived sharpness.
However, on reading the original question again, it is clear that the shadows from the hairs on his arms have nothing to do with the pinhole, they are simply cast by the sun.
The sharpness of any shadow depends on the size of the light source. With an overcast sky, the light source is very big, and shadows disappear altogether.
A clear sky will give fairly sharp shadows, and because the eclipsing moon reduces the size of the illuminating sun, the shadows will sharpen further. This won鈥檛 happen evenly, however, because the partly occluded sun is no longer circular but crescent-shaped.
Just before totality is reached (when the eclipse鈥檚 鈥渄iamond ring鈥� is visible), the sun will approximate a point source and the shadows will be at their maximum sharpness.
Julian Money-Kyrle, London, UK
鈥� It is possible the question was misunderstood. The shadows of the hairs were not formed with the light of a pinhole, but directly from the almost-total eclipse.
When the eclipse is getting near totality, the visible part of the sun is a thin crescent. If the crescent has become just a short arc, then shadows cast by it will be smeared in the long direction but not in the crosswise direction.
So hairs that go in the same direction as the arc will cast very thin, sharp shadows, whereas hairs that are perpendicular to it will cast shadows that are vague.
You can see similar sharp shadows when they are cast by light reflecting off a curved surface of a car. The image of the sun on the car is very small, much smaller than the sun in the sky, so it casts much sharper shadows.
Eric Kvaalen, Les Essarts-le-Roi, France
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